This invention relates to the precision direction finding of radio frequency emitting targets, typically a surface-to-air missile launching platform, and more particularly to an improved means and method for the rapid location of the target in terms of GPS coordinates of the target by its transmission from a single airborne platform for use by the platform or relay to other aircraft to allow engagement of the target with GPS guided weapons.
The ability to quickly locate a surface-to-air missile system and attack that system in a xe2x80x9csure killxe2x80x9d lethal suppression of enemy air defense capability is a combat task of high priority. Increased mobility surface-to-air missile systems, able to position, engage, and fire at an airborne target and quickly move from the location in which a missile is discharged have placed added emphasis on the ability to quickly locate and engage these surface-to-air missile system targets before they can move. Current methods employing phase difference interferometry in an anti-radiation signal processing scheme, are limited in their ability to engage these targets.
The current primary suppression of enemy air defense concepts employing AGM-88 high speed computer guided anti-radiation missiles [HARM} have several shortfalls, including:
a) difficulty in logistically supporting the HARM system;
b) limited automatic or automated mission planning capability;
c) difficulty in simulating decoys and saturating enemy threat radars without putting friendly forces in danger;
d) limited ability to reactively target surface-to-air threats; and
e) limited ability to employ off-board targeting sources in a timely way to provide accurate location of the target in a GPS ground grid. This is particularly true when these off-board sources locate mobile surface-to-air missile systems targets.
The most significant limitation of the current suppression of enemy air defense weapons is that they all depend on RF homing for guidance, and are vulnerable to enemy emission control counter measure tactics. The standard defense employed by the enemy is to cease its RF target acquisition, and track radar active emissions, or to xe2x80x9cshut downxe2x80x9d to avoid engagement. These include both tactics associated with a single missile launch site, as well as netted emission control concepts. The angular resolution of the anti-radiation phase difference interferometer on the HARM is insufficient to determine the location of the target with sufficient accuracy to allow the target to be engaged by a GPS guided missile such as a modified HARM or a dedicated GPS guided weapon as part of the engagement sequence using anti-radiation homing phase difference interferometry.
Advanced tactical targeting concepts are focused on the use of multiple air vehicles operating in triangulation modes with communication, via data links, between the various air vehicles and a ground or airborne controller, to enable a remitting target to be located with a high degree of accuracy in the GPS grid. This concept, because of the number of air vehicles involved, and the large number of data links required, as well as the need for a station controller used to monitor these data links, is extremely expensive and complicated to implement. It is usually associated with significant operations deficiencies related to the complexity of the approach.
Thus, a technique is required which includes allowing a single air vehicle, such as an unmanned air vehicle or xe2x80x9cUAVxe2x80x9d, for example, to incorporate ultra high precision direction-finding capabilities. This technique would enable a single UAV to locate emitting targets in the GPS coordinate system within a very short time, typically in less than a second, after the emitting target has come up on-line and commenced to broadcast. Typically, such vehicle could be a Dragon Drone now in use by the Navy Marine Corps. (FIG. 1).
GPS-updated inertially guided weapons have the ability to engage targets located in the GPS grid to an accuracy of, typically, five meters, or better to use this unique satellite guidance concept, it is necessary to locate the target in the GPS grid to an accuracy of the same order from platforms located at ranges as great as fifty kilometers from the target. Currently, no known techniques are available to do this.
Briefly stated, the invention enables the location of emitting ground targets in the GPS coordinate system with high precision by measuring the angle, angle direction to the ground target relative to the angle, angle bearing from the air vehicle to the GPS satellites being employed for air vehicle navigation. It does so by processing the signals received from emitting ground targets, such as surface to air missile systems, and signals received from the GPS satellite being employed for air vehicle navigation in such a way to impress a Doppler frequency shift on the signals proportional to their direction cosine of arrival. This in turn enables the measurement of the bearing to the emitting ground target relative to the non-orthogonal coordinate system formed by the at least four, but as many as ten, GPS satellites being used to navigate the air vehicle. Knowing the position of the air vehicle in GPS coordinates and the angle bearing to the emitting target allows the position of the emitting target to be determined in the local GPS grid map to a high accuracy in GPS latitude, longitude and elevation. The signal processing technique is also shown to have application in making the air vehicle GPS receiver immune to single and multiple GPS jammers by separating the GPS signals received by the air vehicle from the GPS satelites from the jammer signals based on their direction of arrival. It can accomplish this using the same processing logic to locate emitting ground targets by separating the GPS signals from jammer signals based on their direction of arrival. This is accomplished by knowing the Doppler frequency shifts associated with the GPS satellite signals and using these frequencies to separate the jammer signals which have different Doppler frequency shifts since they have different directions of arrival than the GPS satellite signals.
In the U.S. patent to Schneider, U.S. Pat. No. 5,432,520, there is disclosed a synthetic aperture radar used with a global positioning inertial navigation system on a moving aircraft.
The U.S. patent to McIntosh, U.S. Pat. No. 6,232,922, discloses the use of angle of arrival of a received signal, but not necessarily as part of the disclosed method.